Recent research has shown that the acquisition of a second cue–outcome association can interfere with responding appropriate to a previously acquired association between another cue and the same outcome, even if the two cues had never received compound training (Matute & Pineño, 1998a). This is similar to other results in the paired-associate literature but it is problematic for associative theories of learning because all of them assume that compound training is necessary for cues to interfere with each other. However, given several assumptions, a recent revision of Wagner's (1981) SOP model proposed by Dickinson and Burke (1996) could account for most of the data available on interference between elementally trained cues. According to the modified SOP model, the target cue that is paired with the outcome during Phase 1 could acquire an inhibitory association with the outcome during the Phase 2 trials in which the interfering cue is trained and the target cue is absent. This inhibitory association could be responsible for the weak responding observed to the target cue during testing because it could interfere with the excitatory association acquired during Phase 1. If this is true, interference should be weaker as the number of Phase 2 interfering trials is reduced. However, the three experiments reported here show that interference can occur even when only one interfering trial is given during Phase 2. The results of these experiments, along with other results in the literature, add support to the idea that interference between elementally trained cues occurs during retrieval and that it is not due to the formation of inhibitory associations between an absent cue and the outcome.